Add release instructions doc
This MR adds a RELEASE.md with step-by-step instructions for finalising and publishing a release.
Maintenance will be performed on git.ligo.org, containers.ligo.org, and docs.ligo.org on Tuesday 30 April 2024 starting at approximately 9am PDT. It is expected to take around twenty minutes and there will be several periods of downtime throughout the maintenance. Please address any comments, concerns, or questions to the helpdesk.
This MR adds a RELEASE.md with step-by-step instructions for finalising and publishing a release.